Env class confusion

I'm just wrapping up my very first esri UC and I'm trying to get my feet wet (finally) with some python coding. I have what I hope is a fairly straight forward question about the env class. I've stolen some snippets of code and played around with it a bit but can't figure out how this class works exactly. I realize it basically draws in the environment settings of something, but I'm confused exactly what that means!

I had a demo code that buffered a few feature classes in a file geodatabase. So my env line looked something like arcpy.env.workspace = "pathname.gdb" I could then manipulate the files in the GDB and use geoprocessingtools on them. Why does entering something like arcpy.env.workspace = "pathname.mxd" not work? Can you only get environments from geodatabases?

Sorry for the newbish questions, I searched the forums for an answer but couldn't find anything on it. Any help is appreciated!